1. Field of the Invention
The present invention relates to a cell balance device for achieving a cell balance of secondary batteries connected in series and to a battery system including the cell balance devices, and more particularly, to a cell balance device capable of preventing a switch circuit of the cell balance device from being broken down by an overcurrent and to a battery system including the cell balance devices.
2. Description of the Related Art
FIG. 5 illustrates a circuit diagram of a conventional cell balance adjusting circuit. A cell balance device including the conventional cell balance adjusting circuit is provided with a battery pack portion, in which a plurality of secondary battery cells (hereinafter, referred to as cells) 401 to 406 as basic components are connected in series, and switches 411 to 462 each having one contact connected to a node between the cells. The other contact of each of the switches 411, 421, 431, 441, 451, and 461 is connected to one electrode of a capacitor 407 which is a voltage hold device. The other contact of each of the switches 412, 422, 432, 442, 452, and 462 than the one contact on the cell side is connected to the other electrode of the voltage hold device 407. A load circuit or a charging circuit 408 is connected across the battery pack.
Open/close signals for the respective switches are connected so that the switch 411 and the switch 412 may operate simultaneously. Signals are similarly connected so that the switch 421 and the switch 422, the switch 431 and the switch 432, the switch 441 and the switch 442, the switch 451 and the switch 452, and the switch 461 and the switch 462 may be open/closed simultaneously each as a switch of a set of corresponding two switches.
Further, the switch open/close signals sequentially turn ON/OFF the switches 411 to 451 and the switches 412 to 462. Then, after finishing turning ON/OFF the switch 461 and the switch 462, repetitive switch open/close operations are continuously performed by returning to the first ON/OFF operation of the switch 411 and the switch 412.
Next, an operation of the conventional cell balance adjusting circuit is described. The conventional cell balance adjusting circuit scans switches sequentially in one direction so that the switches are switched over while forming parallel connections between the series-connected adjacent cells and the voltage hold device 407.
The switches 411 and 412 are configured to receive signals for simultaneous open/close operation, and the switch 421 and the switch 422 are also configured to be open/closed simultaneously. The same is applied to the other switches, and the simultaneous open/close operation is performed in each combination of the switch 431 and the switch 432, the switch 441 and the switch 442, the switch 451 and the switch 452, and the switch 461 and the switch 462.
The switches are controlled in this way, to thereby form parallel connections sequentially between all the series-connected cells and the voltage hold device 407. After completion of the formation of parallel connections to all control target cells in the battery pack, the same switching operation is repeatedly performed from the first cell, thereby adjusting a cell balance.
In the conventional technology, however, when an overcurrent flows through the switch, the switch is broken down, and hence there is a problem in that the reliability of the cell balance device is lowered.